Castor oil is composed of triglycerides of fatty acids, of which 85% to 95% consist of ricinoleic acid. In the presence of methanol, the ester predominantly obtained by transesterification of castor oil is methyl ricinoleate (or methyl 12-hydroxy-cis-9-octadecenoate). This compound is used, inter alia, as a starting material in the production of 11-aminoundecanoic acid, a constituent monomer of Rilsan® 11, which is a polyamide with exceptional physical properties, developed by the applicant.
During the production of 11-aminoundecanoic acid, methyl ricinoleate is subjected to gas-phase thermal cracking. To this effect, it must contain a minimum amount of glycerides, i.e. of tri-, di- and monoglycerides, since these products are very difficult to vaporize, and often break down before vaporization, which results in a lowering of the selectivity of the cracking. Similarly, the methyl ricinoleate must contain a minimum amount of ricinoleic acid, which is itself also difficult to vaporize.
It is therefore desirable to have a method which makes it possible to carry out the most complete transesterification possible.
Many processes for the transesterification of plant oils are known. In order for the reaction to be considered complete, it is necessary to use excess alcohol. In order to avoid too great a consumption of alcohol, the transesterification reaction can be carried out in two steps:                a first transesterification step is carried out in the presence of an excess of light alcohol and of an acid or basic catalyst, and then the glycerol formed is extracted from the reaction mixture in order to shift the equilibrium of the reaction toward the formation of ester of the light alcohol;        the organic phase recovered at the end of this first step is again treated with alcohol such that a transesterification yield close to 100% is finally obtained.        
Such a process is, for example, described in document U.S. Pat. No. 5,354,878, which discloses a transesterification process which admittedly makes it possible to obtain a very high conversion of plant oil to fatty acid esters, but which requires four transesterification steps with four separations of the glycerol formed. More specifically: a first transesterification step is carried out in a first reactor 14 in the form of a column, by introducing the reaction mixture containing the plant oil, methanol and NaOH into the top of the column, at a flow rate which is lower than the settling rate of the glycerol which is eliminated at the bottom of the column, and then the reaction mixture is transferred into a second reactor 20 and the trans-esterification is continued according to a second step without the addition of reactants; washing with water is then carried out, followed by a third trans-esterification step carried out in a third reactor 36 (with the addition of alcohol and of catalyst), and then the glycerol is separated in the same way as during the first transesterification reaction, and the reaction mixture is transferred into a fourth reactor 40 where a fourth transesterification step is carried out without the addition of reactants; the latter reaction mixture containing the transesterification product is subjected to washing with water and then to drying. The equipment required for implementing this laborious process is complex, producing increased production costs.
Moreover, although the transesterification process described in that document functions well with rapeseed oil or another oil, for example with a sunflower oil, it has been found that it is very disadvantageous to implement it with a hydroxylated plant oil such as castor oil. This is because, on a castor oil/methanol/basic catalyst reaction mixture, the rate of settling out of glycerol is 5 to 20 times slower than on the corresponding mixture with a rapeseed or sunflower oil. In order to implement the process described above with a column reactor such that the flow is sufficiently slow for the glycerol to separate from the reaction mixture during the first transesterification step, it would be necessary to have a column with a gigantic diameter.
Document U.S. Pat. No. 5,399,731 describes another process for the transesterification of fatty acid triglycerides with a light alcohol and in the presence of a basic catalyst, said process comprising one or more transesterification steps and also a step of adding water or a dilute organic or inorganic acid to the ester phase obtained after the separation of the glycerol phase. As indicated in column 4, lines 11 to 16, the addition of water (carried out after the second or the final transesterification) makes it possible to eliminate, from the ester phase, catalyst residues and other impurities. This process is considerably simpler than the previous one, which makes it possible to drastically reduce production costs. However, when an attempt is made to implement this transesterification process with castor oil, regardless of whether it is with a single or several successive transesterification steps, an ester fraction which contains too many residual glycerides to be suitable for use as a biofuel or as a starting material in the production of Rilsan® 11 is obtained.
Other documents describe transesterification processes applied to castor oil. The conversion of the oil to esters, obtained by means of these processes, does not however exceed 94%.
Document GB 566 324 describes a process for the transesterification of castor oil in the presence of methanol and a basic catalyst. According to example 2, this process comprises a first transesterification step, followed by a step of separating the lower phase, rich in glycerin, by settling out. Several variants are subsequently described for processing the fatty ester-rich upper phase. According to a first variant (example 2.a), the upper phase is washed three times in water, resulting in a glycerin recovery rate of 78%. According to a second variant (example 2.b), said upper phase is subjected to a second transesterification step in the presence of methanol and a basic catalyst, and then to an acidification step. After elimination of the excess methanol, the glycerin-rich phase is separated by settling out, resulting in a glycerin recovery rate of 86%. According to a third variant (example 2.c), said upper phase is subjected to a second and then to a third transesterification step in the presence of water, with subsequent separation of the glycerin by settling out. The conversion obtained, measured by the glycerin yield, is 94%.
The publication by Agra I. B. et al. (Renewable Energy, Pergamon Press, Oxford, GB, vol. 9, no. 1, Sep. 12, 1996, pages 1025-1028) describes a process for the transesterification of castor oil which is performed in two steps in the presence of methanol and sulfuric acid. After a first transesterification step, the reaction medium is neutralized by means of a sodium hydroxide solution, and sodium chloride is added in order to assist with the separation of the glycerin. The upper phase which results therefrom is subjected to a second transesterification step, resulting in a glycerin recovery rate of only 82%.
The present invention intends to remedy the drawbacks exhibited by the abovementioned transesterification processes. It aims to propose a process which is particularly suitable for the transesterification of hydroxylated oils, in particular castor oil. The objective of the present invention is therefore to provide a process for producing methyl or ethyl esters of castor oil which makes it possible to achieve a very high conversion to esters, while at the same time being carried out at moderate temperatures and pressures, and requiring only a moderate number of steps.